The present invention is directed to a ceramic soldering device for soldering with resistance heating, particular for the reflow soldering of electronic components that usually have a multitude of terminal legs.
As a rule, a defined force is exerted on the terminal legs of electronic components via a soldering device employed in the soldering equipment. The pressing of the soldering device and the heat transmission for the soldering process or, respectively, the production of the necessary heat usually occurs on the basis of the use of a soldering element. The standard embodiments are thereby executed so that the terminal legs of the electrical component are arranged side-by-side in a row and are pressed and heated by an edge or line-shaped soldering element, for example a U-shaped part having a long transverse web or bite portion. When the soldering element is of an electrically conductive material and the heat is generated by resistance heating in the soldering element, then there is a risk of damage to the electrical component due to the voltage drop-off or difference along the soldering element that will occur between the different terminal legs. The current generated in the component, as a result thereof, can have a disadvantageous consequence.
The notoriously utilized technique of reflow soldering with a soldering element has been improved for avoiding the above-described disadvantages. For example, work can be carried out with a low voltage in order to avoid a voltage difference between the individual electrical terminals of the component. The course of development embarked upon, however, has not favored this low-voltage technique, but has attempted to manage voltage drop-off or differences along the soldering element by utilizing different materials.
In detail, the reduction of the voltage differences or drop-off between various terminal legs of an electronic component during soldering can be reduced by the following measures, in addition to being reduced by the utilization of a low-voltage technique:
A. The material of the soldering element has a low ohmic resistance and the soldering element is divided into a plurality of soldering elements that are electrically connected in parallel. In this version, adequately large conductor cross sections can be utilized.
B. A second feature is that the soldering element can be manufactured of a sheet metal which is bent into a U-shape. The width of such a soldering element in the downward direction is limited by the thickness of the sheet metal and by the necessary bending radius. With this version, however, the voltage differences between different terminal legs can be eliminated by contacting a respective U-shaped soldering element to a terminal leg.
Developments that have taken the limited integration space of the soldering element into consideration have provided a coating with a ceramic for electrical installation of the soldering elements or devices at their contacting surfaces in the direction of the terminal legs. Simple coatings, however, harbor the risk that the ceramic layer will flake off, due to different coefficients of thermal expansion at the soldering element and at the ceramic layer.
European Published Patent Application 0 357 977, which claims priority from German Patent Application P 38 30 407, discloses a soldering element that comprises a ceramic strip or, respectively, a ceramic rod, which is glued into a channel of a soldering element. The contact between the terminal legs and the soldering element occurs via this ceramic strip. As a result of the low ohmic resistance of this soldering element, high electrical currents must flow through the soldering element in order to produce the necessary quantity of heat. This requires an involved switching and control technology which also requires an involved power supply. An extremely low-resistance connecting technique, further, is needed, given high currents. What is desirable here is the principle that the heat should arise in the soldering element and not at the electrical terminals of the soldering element. In order to exclude disadvantages in this respect to the greatest possible degree, planar ground and gold-plated contact surfaces are sometimes utilized at the expanded portions of the soldering element. An attempt has been made with temperature sensors in the form of thermal elements, which are additionally soldered on to monitor or control the temperature along the soldering element edge.
IBM Technical Disclosure Bulletin, Entitled "Self-Levelling Soldering Tool", Vol. 11, No. 8, January 1969, p. 1026, discloses a fully ceramic soldering element that has embedded resistant wires for generating heat. The mechanical holding occurs at the upper part of this ceramic element.